Monday, June 30, 2008

What to notice: The patterns in the four squares are stationary, yet they appear to move.

Brief Comment: This week’s illusion is a motionless image that creates the perception of motion. In this respect, the illusion is similar to the Rainbow Boom/Rainbow Bust illusion that I posted a few weeks ago. People differ in how they see peripheral drift illusions: for some people, the effect is very strong, and others do not see it at all. The effect seems to become less strong as you age.

This week’s illusion is based on a design principle made famous in Akiyoshi Kitaoka’s “rotating snakes.” To make this type of illusion, you start with an individual element made of segments that follow the pattern “dark, bright, less bright, darkest” (or, to put it into numbers, the luminance levels of the segments follow a pattern something like 0.3, 1.0, 0.7, 0.0—there are nuances concerning the luminance ratio and the size of segments). You then repeat the element enough times to form a ring. The effect appears to become stronger if you include multiple rings near the same location.

These patterns are fun to manipulate in programs like Adobe Illustrator and Adobe Photoshop. Here, I have created a pattern and placed the resulting rings into a four-square configuration with bright colors (Kitaoka-like elements in an Andy Warhol-like display with Keith Haring-like colors).

Why does this effect occur? Most explanations of peripheral drift illusions suggest that the illusory patterns fool the eye into sending to the brain information that mimics a real motion signal.

The eye sends information to the brain through different types of neurons. Some of these neurons transmit information faster than others, and some of these neurons respond more quickly to high-contrast parts of an image than to low-contrast parts of the image. Because of these differences, the response of the eye to one part of the illusory pattern reaches the brain at a slightly different time than the response of the eye to another part of the illusory pattern. The difference in the arrival time is exactly the same type of event that would occur with “real” motion, and so motion detectors in the brain signal that motion has occurred.

The details of these events for the peripheral drift illusions have not yet been settled. One explanation is based on the idea that the early visual system responds faster to high-contrast information than it does to low-contrast information. Variations on this idea were put forward by Ben Backus and Ipek Oruc (here is a link to their paper) and by Bevil Conway, Akiyoshi Kitaoka, Arash Yazdanbakhshm, Christopher Pack, and Margaret Livingstone (here is a link to their paper). Another explanation suggests that the motion occurs because the neural systems that respond to positive contrast changes (ON channels) respond faster than the neural systems that respond to negative changes in contrast (OFF channels). This idea was put forward by Maria Del Viva, Monica Gori and David Burr to explain a slightly different illusion (here is a link to their paper).

I will be on the road in California most of July. I will try my best to keep up with the blog while traveling.